It is known to provide a tractor with a suspended front axle, in which the axle is connected to the engine and transmission tunnel (which constitutes the chassis of the tractor) by means of a panhard rod and a suspension unit that comprises a hydraulic jack with a double acting piston. An example of such an existing hydraulic circuit and hydraulic jack is shown in FIG. 1 of the accompanying drawings. In FIG. 1, the jack 10, has a cylinder body 12, a piston 14, a piston rod 16 and two hydraulic working chambers 18 and 20 on opposite sides of the piston 14. Anchoring blocks 22 and 24 are provided on the cylinder body 12 and the piston rod 16 for connecting the hydraulic jack 10 to the chassis and the panhard rod of the tractor, respectively. Extending or retracting the jack 10 has the effect of varying the distance between the front axle and the chassis.
The two working chambers 18 and 20 of the hydraulic jack 10 are connected by means of flexible hydraulic hoses 26 and 20 to a remotely located valve block 30, shown in dotted lines in the drawing. The valve block 30 comprises two solenoid operated two-port valves 32 and 34 each of which acts as a non-return valve when not energized and establishes a direct connection between its two ports when energized. The valve block 30 has four connectors, namely connectors 36 and 38 for the hoses 26 and 28 leading to the hydraulic jack 10, a connector 40 for a line leading to an accumulator 42 and a connector 44 for a supply/feed line 46 from a second valve block (not shown in the drawing). The second valve block is connected to a pressure line and a return line and acts to pressurize the line 46 when the suspension is to be raised and to relieve the pressure in the line 46 when the suspension is to be lowered. When no adjustment is to be made to the height of the suspension, the line 46 is simply shut off and during such time the valves 32 and 34 in the valve block 30 are controlled to act in conjunction with the accumulator 42 to cushion and damp movements of the front axle so as to reduce the shocks transmitted to the tractor chassis. As the hydraulic circuit is itself known, its operation will be clear to the person skilled in the art and it need not therefore be described in greater detail within the present context.
A problem is encountered in the front axle suspension system of existing tractors on account of the remote location of the valve block 30 from the hydraulic jack 10. The long hoses 26 and 28 connecting the valves 32 and 34 to working chambers of the jack 10 tend to flex and stretch when placed under pressure. Their resulting change in volume introduces a sponginess that reduces the response and performance of the suspension system.